1. Field of the Invention
The present invention relates to a semiconductor devise and, more particularly, to a method for forming a conductive line of a semiconductor device which enhances thermal stability and ensures low electric resistance.
2. Background of the Related Art
In forming a conductive line of tungsten/silicon structure, tungsten and silicon react with each other at a temperature above 600xc2x0 C. and tend to form tungsten silicide. The tungsten silicide has electric resistance ten times as high as that of tungsten, so that it is difficult to use the tungsten silicide as a material for a conductive line of a semiconductor device. Furthermore, it requires a reaction prevention film between tungsten and silicon because silicon is destructed when the silicide is formed.
A conventional method for forming a conductive line of a semiconductor device will be explained with reference to the attached drawings.
FIGS. 1a to 1d are cross-sectional views showing a conventional method for forming a conductive line formed with tungsten/reaction prevention film/silicon structure, and FIGS. 2a to 2e are cross-sectional views showing another conventional method for forming a conductive line.
Process steps of forming a conductive line of a tungsten/reaction prevention film/silicon structure will be explained with reference to FIGS. 1a to 1d. 
Firstly, a gate insulating film 12 is formed on a semiconductor substrate 11, and a semiconductor layer 13 for forming the conductive line is formed thereon, as shown in FIG. 1a. At this time, polysilicon may be used for the semiconductor layer 13. Referring to FIG. 1b, a reaction prevention film 14 and a tungsten film 15 are sequentially formed on the semiconductor layer 13. The reaction prevention film 14 is made of a material which doesn""t react with the tungsten film 15 and the semiconductor layer 13, such as nitride of transition metal and refractive metal, or oxide having electric conductivity.
Thereafter, a tungsten film 15, a reaction prevention film 14 and a semiconductor layer 13 are selectively etched by photolithography to form a conductive line 16, as shown in FIG. 1c. Referring to FIG. 1d, impurity ions are implanted on an entire surface of the substrate using the conductive line 16 as a mask, and then the implanted ions are drive-in diffused to form source and drain regions 17 within the surface of the semiconductor substrate 11 on both sides of the conductive line 16. The reaction prevention film 14 in a tungsten layer/reaction prevention film/semiconductor layer structure of the conductive line restrains reaction between the semiconductor layer 13 and the tungsten film 15.
The process of forming the conductive line, as described above, is performed at a low temperature. Thus, thermal stress given to lower layers placed below the conductive line is not so high and impurity distribution in semiconductor substrate 11 is rarely varied.
Process steps of forming a conductive line at a high temperature which is performed in a reversed way of the above steps will be explained.
FIGS. 2a to 2e shows process steps of forming a conductive line of a denudation W/Si structure. First, a gate insulating film 12 is formed on a semiconductor substrate 11, and a semiconductor layer 13 for forming a conductive line is formed thereon, as shown in FIG. 2a. At this time, the semiconductor layer 13 is formed with polysilicon.
Referring to FIG. 2b, a tungsten nitride film 21 is formed on the semiconductor layer 13. Referring to FIG. 2c, the substrate on which the tungsten nitride film 21 is formed is heat-treated at 1000xc2x0 C. to form a reaction prevention layer (not shown) of silicon nitride between the tungsten nitride film 21 and the semiconductor layer 13. Simultaneously, the tungsten nitride film 21 is changed into a tungsten film 21a. 
Then, the tungsten film 21a and the semiconductor layer 13 are selectively etched by photolithography to form a conductive line 22, as shown in FIG. 2d. Referring to FIG. 2e, impurity ions are implanted on an entire surface of the substrate using the conductive line 22 as a mask, and the implanted ions are drive-in diffused to form source and drain regions 17 within the surface of the semiconductor substrate 11 on both sides of the conductive line 22.
In the aforementioned process of forming the conductive line, the tungsten nitride film 21 is deposited and heat-treated on the semiconductor layer 13, and the tungsten nitride film 21 which is unstable at a high temperature is changed into the tungsten film 21a during the heat treatment. Simultaneously, nitrogen contained in the tungsten nitride film 21 and silicon contained in the semiconductor layer 13 are combined, to form silicon nitride at the interface of tungsten layer 21a and the semiconductor layer 13. This silicon nitride restrains reaction of silicon and tungsten even at a high temperature above 1000xc2x0 C. As described above, though the conductive line of denudation W/Si structure has a high electric resistance in a deposited state, its resistance after a heat treatment at a high temperature executed thereto is similar to that of the conductive line having tungsten/reaction prevention film/silicon structure.
Accordingly, the denudation W/Si structured conductive line restrains reaction of tungsten and silicon, and obtains satisfactory thermal stability without employing deposition of a reaction prevention film.
However, the related art method for forming a conductive line of a semiconductor device has the following problems.
Firstly, in forming a conductive line with a tungsten/reaction prevention film/silicon structure in which a reaction prevention film is formed to restrict reaction of tungsten and silicon, an additional deposition process of the reaction prevention film is required. Furthermore, the resistance of the reaction prevention film is greater than that of tungsten so that the electric resistance of a gate is increased, thereby deteriorating operation characteristics of a device.
Secondly, the formation of the reaction prevention film affects the electric conductivity of the tungsten film formed thereon, so as to worsen electric characteristic of a conductive line.
Thirdly, since the conductive line is formed at a low temperature, thermal stability may not be ensured in the subsequent heat treatment process.
Additionally, in forming a denudation tungsten/silicon structured conductive line in which a tungsten nitride film is formed on silicon, the reaction prevention layer of silicon nitride is formed by a heat treatment at a high temperature. Accordingly, considerable thermal stress is applied to the device.
Furthermore, impurity distribution of the semiconductor substrate is changed during the heat treatment in a high temperature, characteristics of the device is deteriorated.
Accordingly, the present invention is directed to a method for forming a conductive line of a semiconductor device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for forming a conductive line of a semiconductor device which has a low electric resistance and high thermal stability.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for forming a conductive line of a semiconductor device includes the steps of forming an insulating layer on a semiconductor substrate, sequentially forming a semiconductor layer and a tungsten film on the insulating layer, nitrifying the tungsten film with heat treatment, and selectively etching the tungsten film and the semiconductor layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.